Calcium signal communication in the central nervous system

Biology of the Cell 96 (2004) 79–91 www.elsevier.com/locate/biocell

Review

Calcium signal communication in the central nervous system Katleen Braet a, Liesbet Cabooter a, Koen Paemeleire b, Luc Leybaert a,* a

Department of Physiology and Pathophysiology, Ghent University, De Pintelaan 185, B-9000 Ghent, Belgium b Department of Neurology, Ghent University Hospital, De Pintelaan 185, B-9000 Ghent, Belgium Received 14 October 2003; accepted 30 October 2003

Abstract The communication of calcium signals between cells is known to be operative between neurons where these signals integrate intimately with electrical and chemical signal communication at synapses. Recently, it has become clear that glial cells also exchange calcium signals between each other in cultures and in brain slices. This communication pathway has received utmost attention since it is known that astrocytic calcium signals can be induced by neuronal stimulation and can be communicated back to the neurons to modulate synaptic transmission. In addition to this, cells that are generally not considered as brain cells become progressively incorporated in the picture, as astrocytic calcium signals are reported to be communicated to endothelial cells of the vessel wall and can affect smooth muscle cell tone to influence the vessel diameter and thus blood flow. We review the available evidence for calcium signal communication in the central nervous system, taking into account a basic functional unit –the brain cell tripartite- consisting of neurons, glial cells and vascular cells and with emphasis on glial-vascular calcium signaling aspects. © 2003 Elsevier SAS. All rights reserved. Keywords: Neuron; Glial cell; Brain vessel; Neuroglial; Neurovascular; Gliovascular; Neurobarrier; Gap junctions; Paracrine signaling.

1. Introduction Traditionally, neuronal electrochemical impulses are seen as the basis of information processing in the central nervous system (CNS). There is, however, growing evidence that so called “non-excitable” cell types such as glial cells and also brain vessel cells, are actively participating in brain functioning by responding to synaptic activity (Dani et al., 1992), by modulating these information circuits (Nedergaard, 1994; Kang et al., 1998) and by exchanging signals to coordinate the functioning of the basic triad consisting of neurons, glial cells and microvascular cells. Intracellular calcium ions play a pivotal role in non-excitable cells as a messenger for intracellular and intercellular signaling (Sanderson et al., 1994; Berridge et al., 2000; Rottingen and Iversen, 2000). Because of a certain degree of homology between action potentials and calcium transients these signals provide non-excitable cells with some form of ‘calcium excitability’ (Verkhratsky et al., 2002). Intercellular calcium signals are transient changes in cytoplasmic free calcium that are, analogously to action potentials in neurons, characterized by an initiating * Corresponding author. E-mail address: [email protected] (L. Leybaert). © 2003 Elsevier SAS. All rights reserved. doi:10.1016/j.biolcel.2003.10.007

trigger followed by a mechanism that propagates the calcium signal to neighboring cells. The velocity of signal propagation is in the order of tens of micrometers per second, i.e. two to three orders of magnitude slower than action potential propagation. The spectrum of intercellular calcium signals ranges from the most elemental form of calcium signal exchange between just a pair of cells up to the massive intercellular calcium waves encompassing hundreds of cells. In its elemental form the calcium signal is propagated by diffusion of a messenger, analoguous to electrotonic (subthreshold) signal spread. In principle, the term calcium wave is to be reserved for calcium signal propagation that involves a regenerative aspect, analoguous to action potential signal propagation. This distinction is however not generally made in the literature and the term calcium wave has been frequently applied to denote calcium signals being communicated over a large population of cells regardless of the involvement of regenerative phenomena. The communication of calcium signals between brain cells is a typical feature of glial cells. It is however not restricted to this kind of cells as neurons, also exchange calcium signals among each other (Yuste et al., 1995; Charles et al., 1996) as well as with the surrounding astrocytes (Dani et al., 1992; Charles, 1994). Extensive research over the past

80

K. Braet et al. / Biology of the Cell 96 (2004) 79–91

ten years has identified several calcium signal communication systems in the brain and has in addition put forward possible roles or functions. In this paper we review the data available on cell-to-cell communicated calcium signals in the central nervous system. We focus on the various pathways involved in the cell-to-cell propagation of calcium signals and highlight the role of these signals in communication systems such as neuroglial and gliovascular signaling.

2. Calcium signal communication mechanisms Two mechanisms have been identified that support cell-tocell communication of calcium signals in various cell types. The first mechanism involves the diffusion of a calcium mobilizing messenger, or calcium itself, through gap junction channels (Boitano et al., 1992). The second mechanism relies on paracrine signaling (Osipchuk and Cahalan, 1992; Hassinger et al., 1996) involving the release of a messenger, diffusion in the extracellular space, binding to receptors on neighboring cells and activation of downstream signaling cascades that ultimately lead to an increase of cytoplasmic free calcium in the target cell. The calcium increase in the target cell might on its turn initiate a new cycle of calcium signal communication, i.e. cause re-generation of the cellcell calcium signaling event producing regenerative calcium signal propagation (Domenighetti et al., 1998; Hofer et al., 2002) thereby spreading the signal over a larger population of cells as compared to the purely diffusive spread of calcium or calcium mobilizing messengers between cells. 2.1. Gap junctional calcium signal communication Gap junctions are the sites of direct cell-to-cell communication, facilitating the exchange of chemical and electrical signals between cells. These intercellular channels are composed of two hexameric half-channels, called connexons or hemichannels (Goodenough et al., 1996). The end-to-end interaction or ‘docking’ of two connexons, each provided by one of the two neighboring cells, generates an intercellular aqueous channel that allows the exchange of nutrients, metabolites, ions and small molecules with a molecular weight up to ~1000 Dalton (single channel conductance in the order of 100 pS). Gap junctions are ubiquitous throughout the central nervous system (Dermietzel and Spray, 1993; Spray and Dermietzel, 1996; Rouach et al., 2002) and are covered by several recent reviews describing their structure and function (Goodenough et al., 1996; Shibata et al., 2001; Duffy et al., 2002; Evans and Martin, 2002). The importance of gap junction channels in communicating calcium signals between cells has become clear since the direct demonstration that calcium itself and the calcium mobilizing messenger inositol trisphosphate (InsP3) can permeate through gap junction channels (Saez et al., 1989). Subsequent work by others has extensively demonstrated the functionality of InsP3 diffusion through gap junctions as a

calcium signal propagating mechanism (Sanderson et al., 1990; Boitano et al., 1992; Sanderson et al., 1994; Giaume and Venance, 1998; Leybaert et al., 1998). Further work has also demonstrated that the permeability of gap junction channels to InsP3 depends on the connexin subtype being involved (Niessen et al., 2000). An important aspect of calcium signal communication by diffusion of InsP3 through gap junction channels is that even small amounts of InsP3 flowing into a cell, e.g. because of a very low level of gap junctional coupling, are sufficient to produce measurable calcium signals because of the amplification effect associated with calcium-induced calcium release (Braet et al., 2001; Clair et al., 2001; Berridge, 2002). Besides InsP3 and calcium, the calcium mobilizing messenger cADP ribose has also been proven to be a trigger of intercellular calcium signals but its contribution to the intercellular propagation of the calcium signal seems to be dependent on the InsP3-generating activity of PLC (Churchill and Louis, 1998; Leybaert and Sanderson, 2001). Although calcium ions themselves can diffuse through gap junction channels, their role in communicating calcium signals between cells is limited because their diffusion is severely hindered by binding to more slowly diffusing cytoplasmic calcium buffers, which makes the effective diffusion in the cytosol constant much lower as compared to that of InsP3 (Deff Ca2+ = 20 µm2s-1; Deff InsP3 = 300 µm2s-1; Allbritton et al., 1992; Sneyd et al., 1995). Under certain conditions however, calcium can act as an effective calcium signal messenger between cells (Hofer et al., 2001). One such condition is the priming of InsP3 receptors by exposure to slightly elevated background InsP3 concentrations, which brings these receptors in a more calcium-sensitive state (Kaftan et al., 1997). Under these conditions, calcium-induced calcium release, taking place at both InsP3 and ryanodine receptors, brings the cytoplasm into a calcium-excitable state and thus susceptible to stimulation by calcium diffusing in from adjacent cells through gap junction channels (Yule et al., 1996). 2.2. Paracrine calcium signal communication Osipchuk and Cahalan (1992) provided the first evidence that an extracellular messenger mediates the spreading of cell-to-cell calcium signals in mast cells. In principle, every substance being released by a cell and acting on neighboring cells to increase cytoplasmic calcium, is a calcium messenger candidate. However, not all possible candidate messengers (reviewed for glial cells in Verkhratsky et al., 1998) have proven to effectively communicate calcium signals between brain cells. Most evidence is presently available for substances like ATP, glutamate and nitric oxide (NO) (Parpura et al., 1994; Hassinger et al., 1996; Guthrie et al., 1999; Willmott et al., 2000a). The exact type of messenger involved depends on the cell type and paracrine calcium signal communication will therefore be more specifically dealt with in the next chapter where calcium signal communication systems are considered.

K. Braet et al. / Biology of the Cell 96 (2004) 79–91

81

Fig. 1. Schematic representation of calcium signal communication systems in the central nervous system. (1) Presynatically (Pre) released glutamate induces calcium changes in the postsynaptic (Post) nerve ending (Petrozzino and Connor, 1994; Bading et al., 1995; Zamanillo et al., 1999). (2) Postsynaptic NO triggers calcium increases in the presynaptic nerve ending and regulates neurotransmitter release (Reyes-Harde et al., 1999). (3) Release of glutamate and NO from neurons brings about calcium changes in astrocytes (A) (Porter and McCarthy, 1996; Pasti et al., 1997; Matyash et al., 2001). (4) Astrocytes release ATP and glutamate causing calcium signals in neurons (Parpura et al., 1994; Hassinger et al., 1995; Bezzi et al., 1998) and modulating synaptic activity (Araque et al., 1998a; Araque et al., 1998b; Kang et al., 1998; Newman and Zahs, 1998). (5) Bidirectional calcium signal communication has been demonstrated between astrocytes (A) and endothelial cells (EC) in co-culture (Leybaert et al., 1998; Braet et al., 2001). (6) Vasoactive molecules released by endothelial cells through calcium-dependent mechanisms modulate smooth muscle cell (SMC) calcium concentration to produce contraction (Saito et al., 1989) or relaxation (Luckhoff et al., 1988). (7) Smooth muscle cells provide a feedback calcium signal to endothelial cells (Dora et al., 1997). Finally, astrocytic calcium signals dilate small bloodvessels by paracrine messengers acting on smooth muscle cells, either directly or via interposed endothelial cells (Zonta et al., 2003). Neurons are depicted in blue, astrocytes in green and vascular cells in red.

3. Calcium signal communication systems Calcium signal communication is a feature of both neurons and glial cells. Neurons mainly communicate electrical signals between each other, but the exchange of calcium signals is also operative (Yuste et al., 1995; Charles et al., 1996) and is intimately related to synaptic functioning. An increase of calcium by calcium entry through P- and Q-type calcium channels and to a lesser extent through N- and R-type calcium channels is the triggering step of exocytosis in the presynaptic terminal (Meir et al., 1999; Augustine, 2001). Postsynaptic calcium changes are induced at the level of dendritic spines by glutamate acting on NMDA-, AMPAand metabotropic receptors, by voltage-sensitive calcium entry and by release from various dendritic calcium stores (Perkel et al., 1993; Petrozzino and Connor, 1994; Segal, 1995; Bading et al., 1995; Finch and Augustine, 1998; Korkotian et al., 1998; Zamanillo et al., 1999; Augustine et al., 2003). Dendritic spines behave like calcium microdomains connected to the dendritic tree, and intraspine calcium signals regulate synaptic efficacy and plasticity, as demonstrated in phenomena like long-term potentiation and longterm depression (Miyata et al., 2000; Nimchinsky et al., 2002; Segal, 2002), two cellular mechanisms implicated in memory and learning. The field of neuron-neuron communi-

cation of calcium signals per se will not be dealt with in this review. Changes in cytoplasmic calcium concentration form the predominant signal by which glial cells regulate their own activity and influence neuronal behavior (Scemes, 2000). Calcium signals can be communicated among astrocytes, but also between astrocytes and multiple other cell types, including neurons (Nedergaard, 1994; Parpura et al., 1994; Parri et al., 2001), microglia (Schipke et al., 2002) and vascular endothelial cells (Leybaert et al., 1998; Paemeleire and Leybaert, 2000), often appearing as a two-way signal (Fig 1, Table 1). Several recent reviews discuss glial calcium signaling at both the intracellular and the intercellular level (Verkhratsky and Kettenmann, 1996; Scemes, 2000; Kettenmann and Schipke, 2003) and we narrow the focus of the present review to intercellular calcium signal communication aspects. 3.1. Calcium signals between astrocytes Intercellular calcium waves have been described between astrocytes in culture and in organotypic and acute brain slices (Charles et al., 1991; Harris-White et al., 1998; Schipke et al., 2002). Astrocytic calcium waves appear to propagate either via the intracellular gap junctional pathway (Sander-

82

K. Braet et al. / Biology of the Cell 96 (2004) 79–91

Table 1 Calcium signal communication systems

Table 2 Different mechanisms of ATP release

System Neuron-astrocyte

Cell type

Stimulus

Astrocytes

Hypotonicity

Propagating signal Gap junctions Glutamate

References Nedergaard (1994) Porter and McCarthy (1996); Pasti et al. (1997); NO Matyash et al. (2001) Acetylcholine Araque et al. (2002) Astrocyte-neuron Gap junctions Nedergaard (1994) Glutamate Parpura et al. (1994); Hassinger et al. (1995); Bezzi et al. (1998) Astrocyte-astrocyte Gap junctions Charles et al. (1993) ATP Guthrie et al. (1999); Cotrina et al. (2000); Wang et al. (2000) Glutamate Innocenti et al. (2000) NO Willmott et al. (2000a) Astrocyte-microglia ATP Verderio and Matteoli (2001); Schipke et al. (2002) Astrocyte-endothelial ATP and gap junctions Leybaert et al. (1998); cell Paemeleire and Leybaert (2000) Braet et al. 2001 Astrocyte-meningeal ATP and gap junctions Grafstein et al. (2000) cell Endothelial ATP and gap junctions Braet et al. (2003a) cell-endothelial cell

son et al., 1994; Venance et al., 1997) or via the extracellular paracrine pathway (Hassinger et al., 1996; Charles, 1998; Guthrie et al., 1999). One of the major arguments for the role of gap junctions in calcium signal communication between these cells has been based on comparing calcium signal propagation in connexin transfected and wild type glial cell lines (Charles et al., 1992). However, subsequent work from Cotrina et al. (1998b) has demonstrated that connexin transfection is also associated with an increased release of paracrine acting messengers like ATP, thereby fundamentally challenging the interpretation of this kind of experiments. Work with antisense oligonucleotides and with connexin knock-out animals seems to confirm this observation and furthermore demonstrates that changes in connexin expression induces shifts in purinergic receptor subtype expression (Scemes et al., 2000; Suadicani et al., 2003). Much of the interest in the mechanism of calcium signal communication between cells has since then shifted towards investigations on the paracrine pathway. Astrocytes express a myriad of different neurotransmitter receptors (Porter and McCarthy, 1995; Verkhratsky et al., 1998) opening up a large list of candidate extracellular messenger substances. The key extracellular mediator in paracrine calcium signal communication has been identified as ATP (Guthrie et al., 1999). The mechanisms by which astrocytes release ATP appear to be diverse (Table 2). There is evidence for vesicular release (Bal-Price et al., 2002; Volknandt, 2002; Coco et al., 2003) as well as for release through connexin hemichannels, which are half gap junction

Mechanical stimulation UTP Glutamate

Endothelial cells

Low calcium InsP3 Low calcium Mechanical stimulation

Proposed mechanism P-glycoprotein ABC-transporters Hemichannels Vesicular Hemichannels CFTR or CFTR-related Hemichannels Hemichannels Hemichannels Vesicular

References Darby et al. (2003) Ballerini et al. (2002) Stout et al. (2002) Coco et al. (2003) Cotrina et al. (1998b) Queiroz et al. (1999) Arcuino et al. (2002) Braet et al. (2003b) Braet et al. (2003b) Bodin and Burnstock (2001)

channels not connected to neighboring cells (Cotrina et al., 1998b; Stout et al., 2002). This last mechanism has quickly brought the focus of interest back to the connexins, opening up the possibility for their involvement in both gap junctional and paracrine calcium signal communication (Goodenough and Paul, 2003). A major problem concerns the interpretation of work performed with gap junction blockers, because these affect both gap junction channels and connexin hemichannels making it difficult to decide which of both is involved. A promising approach to dissect the two pathways is offered by the use of connexin mimetic peptides, which -when applied over a short time period- only affect connexin hemichannels without influencing the gap junction channels (Braet et al., 2003a; Braet et al., 2003b; Leybaert et al., 2003). Other astrocytic ATP release pathways include the ATP-bindingcassette transporters such as the cystic fibrosis transmembrane regulator (CFTR) or the P-glycoprotein (Queiroz et al., 1999; Ballerini et al., 2002; Darby et al., 2003). The relative contribution of each of these proposed release pathways is currently not known and it is equally unclear whether multiple pathways work together to form a complex cascade of ATP release. Conductive ATP release pathways like connexin hemichannels will indeed be expected to allow, in addition to passing ATP, calcium to enter the cells thereby perhaps activating other release pathways, and autocrine ATP actions might further add to make the sequence of events even more complex. In addition to ATP, ADP might also be involved as a paracrine messenger: ADP is formed by enzymatic (ectonucleotidase) degradation of ATP (Zimmermann et al., 1998) and might in principle also be released by the cells through conductive pathways. Experimental evidence for ADP as a paracrine acting calcium mobilizing messenger is however only available from work in non-glial cells (Moerenhout et al., 2001). UTP is another paracrine messenger candidate in glial cells (Lazarowski et al., 1997; Harden and Lazarowski, 1999). ATP, ADP and UTP all trigger calcium changes in astrocytes and subsequent increases in calcium can trigger ATP release (Cotrina et al., 1998a; Queiroz et al., 1999). ATP release is, however, not always calcium-dependent and varies

K. Braet et al. / Biology of the Cell 96 (2004) 79–91

with the trigger or the release pathway being involved. NOinduced vesicular ATP release (Bal-Price et al., 2002) and low calcium-induced connexin hemichannel related ATP release are reported to be calcium-dependent (Arcuino et al., 2002). NMDA-induced ATP release is dependent on calcium entry whereas AMPA-induced ATP release does not require calcium influx (Queiroz et al., 1999). In addition, calciumindependent ATP release was reported by Wang et al. (2000). Calcium-dependent release will be expected to mediate far reaching regenerative calcium signal propagation while the calcium-independent forms are expected to operate short range calcium signal communication. Adenine and uridine nucleotides act as agonists to increase cytoplasmic calcium in astrocytes (King et al., 2000; Khakh et al., 2001). Purinergic receptors are divided into ionotropic P2X receptors and metabotropic P2Y receptors (reviewed in Ralevic and Burnstock, 1998). P2Y receptors are G-protein linked receptors that increase intracellular InsP3 and mobilize calcium from InsP3-sensitive stores while P2X receptors are ligand-gated cation channels that mediate calcium entry into the cell. P2Y receptors are activated by nanomolar ATP concentrations while micromolar concentrations are needed to activate P2X receptors (James and Butt, 2001). Primary rat cortical astrocytes express all cloned metabotropic P2Y receptors that are activated by both adenine and uridine nucleotides with different potency rank orders, but only some are functionally coupled to calcium increases (P2Y1, P2Y2, P2Y4, and P2Y14) (Zhu and Kimelberg, 2001; Fumagalli et al., 2003). The involvement of purinergic receptors in calcium signal communication has been demonstrated by applying broad spectrum purinergic receptor antagonists (Cotrina et al., 1998a; Guthrie et al., 1999; John et al., 1999). Further work with subtype-selective inhibitors or with selective expression of the receptor subtypes has clearly demonstrated the involvement of P2Y1 and P2Y2 receptors in calcium signal propagation in spinal cord astrocytes and has elucidated their role in conferring distinct charateristics to the calcium wave depending on the receptor subtype involved (Scemes et al., 2000; Koizumi et al., 2002; Fam et al., 2003; Gallagher and Salter, 2003). Astrocytes in primary culture also express various subtypes of ionotropic P2X receptors (P2X2, P2X4, P2X5 and P2X7) of which at least P2X7 has directly been demonstrated to mediate ATP-induced calcium rises (Fumagalli et al., 2003). Studies by Willmott et al. (2000a; 2000b) have demonstrated that NO is able to trigger intercellular calcium waves and is also involved in the cell-to-cell propagation of the calcium signal in mixed glial-neuronal cultures, which contain a majority of astrocytes. NO is synthesized by enzymatic oxidation of L-arginine by nitric oxide synthase (NOS) (Lowenstein and Snyder, 1992) that exists in three isoforms in the CNS: neuronal NOS (nNOS), endothelial NOS (eNOS) also present in astrocytes and microglia (Murphy et al., 1993; Shafer et al., 1998) and inducible NOS (iNOS) in endothelial cells, microglia, astrocytes and neurons (Nathan and Xie, 1994). nNOS and eNOS are constitutively expressed and

83

synthetize NO in response to increases in cytoplasmic calcium levels. NO activates guanylyl cyclase and increases cytoplasmic cGMP which, via a cGMP-dependent kinase, activates ADP-ribosylcyclase to produce cADP ribose (Galione et al., 1993). In astrocytes, cADP ribose triggers ryanodine receptor mediated calcium release (Willmott et al., 2000a), but NO also appears to increase calcium through cGMP-independent mechanisms, either by mobilizing calcium from stores (Bowman et al., 2001; Bal-Price et al., 2002) or by stimulating calcium entry (Willmott et al., 2000b; Matyash et al., 2001). The relative contribution of ATP and NO as paracrine messengers of calcium signals between astrocytes is currently unknown. Next to purinergic and nitrergic messengers, astrocytic calcium waves have also been demonstrated to be associated with a spatially spreading increase in extracellular glutamate (Innocenti et al., 2000). Astrocytic glutamate release is either vesicular (Parpura et al., 1994; Araque et al., 2000; Pasti et al., 2001; Bal-Price et al., 2002), connexin hemichannel related (Ye et al., 2003), anion transporter related (Jeremic et al., 2001) or via P2X7 receptor channels (Duan et al., 2003). Analoguous to ATP release, these mechanisms appear to have different calcium sensitivities: vesicular release and anion transporter related release is calcium-dependent (Parpura and Haydon, 2000; Jeremic et al., 2001) but release through hemichannels or P2X7 channels could not be blocked by intracellular calcium buffering or emptying calcium stores with thapsigargin (Duan et al., 2003; Ye et al., 2003). Calcium-dependency has also been reported for glutamate release triggered by metabotropic glutamate receptor activation (Muyderman et al., 2001) and for prostaglandin-related glutamate release (Bezzi et al., 1998). Glutamate acts on ionotropic and metabotropic glutamate receptors activating calcium entry or calcium mobilization from stores respectively (Pearce et al., 1986; Monaghan et al., 1989; Glaum et al., 1990; Nakahara et al., 1997). Both ionotropic and metabotropic receptor activation triggers calcium responses in hippocampal astrocytes in brain slices (Porter and McCarthy, 1995; Shelton and McCarthy, 1999) and in Bergmann glia in cerebellar slices (Muller et al., 1992; Shao and McCarthy, 1997) (reviewed in Gallo and Ghiani, 2000), opening up the possibility that released glutamate produces on its turn calcium signals and is thus actively involved in the cell-to-cell communication of calcium signals in astrocytes and other glial cells. Calcium signal communication between astrocytes thus seems to rely on many communication systems and messengers, acting either in parallel or displaying regional or cellular specialization (e.g. as demonstrated in Newman, 2001). The role of communicated calcium signals in astrocytes is currently not known, but probably depends on the signaling pathway being solicited and the brain region considered. Several functions have been proposed, either related to the calcium signal or to the messenger involved, including a role in astrocyte proliferation and differentiation (Gallo and Ghiani, 2000; Fam et al., 2003), in clearing neurotransmitters

84

K. Braet et al. / Biology of the Cell 96 (2004) 79–91

and ions from the extracellular space (Newman, 1986; Porter and McCarthy, 1995; Walz, 2000; De Pina-Benabou et al., 2001), in coordinating metabolic reactions (Tsacopoulos et al., 1997), in brain cell volume regulation (Phillis and O’Regan, 2002) and in spreading depression which is one of the models proposed to be involved in the pathophysiology of migraine (Basarsky et al., 1998). 3.2. Calcium signals between neurons and astrocytes Recent data have demonstrated that neurons and astrocytes exchange calcium signals both in vitro as well as in brain slices. The first evidence for communicated calcium signals was provided by Dani and coworkers (1992) who described astrocytic calcium changes in response to neuronal activity in cultured brain slices. Calcium signal communication has also been demonstrated in the inverse direction, i.e. from astrocytes to neurons in co-cultures (Charles, 1994; Nedergaard, 1994; Parpura et al., 1994). Initially it was suggested that astrocyte-neuron calcium signal communication proceeded via gap junctions (Nedergaard, 1994) but a role for such junctions now seems to be reserved for the early stages of brain development (Froes et al., 1999; Rozental et al., 2001). In the adult brain, the topic of astrocyte-neuron gap junctions is still controversial because their density is possibly below the detection limit and functional evidence is difficult to obtain because of the interposed low conductance pathway formed by neurites and astrocyte extensions (as in neuron-neuron coupling - Schmitz et al., 2001). In certain systems however, e.g. the locus ceruleus, neuron-glial gap junctions have been demonstrated in adult brain (AlvarezMaubecin et al., 2000). Further studies show that the bidirectional calcium signal communication system is mainly carried by extracellular messengers released from neurons or astrocytes. Various transmitters such as glutamate, acetylcholine, GABA, ATP or NO, released by neurons through calcium-dependent exocytosis, trigger calcium signals in cortical astrocytes (Pasti et al., 1997), in hippocampal astrocytes (Cornell-Bell et al., 1990; Porter and McCarthy, 1996; Araque et al., 2002) and in Bergmann glia (Matyash et al., 2001). In addition, glutamate and ATP released from astrocytes, also by calcium-dependent mechanisms, trigger calcium changes in neurons in mixed cortical cultures (Parpura et al., 1994), in hippocampal cultures (Hassinger et al., 1995; Araque et al., 2000) and in hippocampal or cortical slices (Pasti et al., 1997; Bezzi et al., 1998). Because the signal flow is bidirectional, it has been proposed that astrocytes are part of a feedback circuit that starts with neuronal activityinduced astrocytic calcium signals, feeding back to the synapse through astrocytic glutamate release that modulates synaptic signal transmission (Pasti et al., 1997; Araque et al., 1998a; Araque et al., 1998b; Kang et al., 1998; Newman and Zahs, 1998). Several recent reviews discuss glial-neuronal signaling in all detail (Araque et al., 1999a; Araque et al., 1999b; Araque et al., 2001; Bezzi et al., 2001; Bezzi and Volterra, 2001; Haydon, 2001; Vesce et al., 2001; Fields and

Stevens-Graham, 2002; Perea and Araque, 2002; Zonta and Carmignoto, 2002; Volterra et al., 2002). Clearly, glutamate exerts a wide range of effects in the brain, either by acting on receptors with subsequent changes in membrane potential or intracellular calcium concentration or by its uptake through specialized astrocytic transporter proteins (GLAST or GLT-1; Voutsinos-Porche et al., 2003) thereby modulating glucose metabolism via changes in intracellular sodium concentration (Pellerin and Magistretti, 1994). This last mechanism has been identified as the trigger for neurometabolic coupling which acts to adapt astrocytic glucose metabolism to the local neuronal demands (Magistretti and Pellerin, 1999). 3.3. Calcium signals between astrocytes and microvascular cells Astrocytes are intermediately positioned between neurons and brain vessels, both in contact with their stellate extensions, and therefore occupy a key signaling position between these two important players (Attwell, 1994; Paspalas and Papadopoulos, 1998). Since neurons can exchange calcium signals with astrocytes, the next question in row is whether astrocytes further propagate this signal to the vessels (Fig. 1). Astrocytes are in contact with smooth muscle cells of arterioles, which determine the vessel diameter and thus blood flow, and with endothelial cells of capillary vessels, which form the blood-brain barrier where important transports take place. Work in astrocyte-endothelial co-cultures has demonstrated that calcium signals can be communicated between astrocytes and endothelial cells in a bidirectional way, making use of both paracrine ATP signaling and gap junctions (Leybaert et al., 1998; Paemeleire and Leybaert, 2000; Braet et al., 2001). Recent work in acute brain slices has demonstrated endothelial calcium responses associated with electrically triggered astrocytic calcium waves (unpublished observation). Further work is needed to clear up the question concerning the presence of gap junctions –even at very low density- between astrocytes and endothelial cells. Paracrine ATP signaling between both cell types is another option as both astrocytes and endothelial cells can be stimulated to release ATP and are both endowed with purinergic receptors (Simard et al., 2003). Similar to astrocytes, endothelial cells release ATP either in a vesicular manner (Bodin and Burnstock, 2001) or via connexin-related mechanisms such as hemichannels (Braet et al., 2003a; Braet et al., 2003b; Leybaert et al., 2003). Brain endothelial cell lines communicate calcium signals between each other via gap junctional and paracrine signaling (Braet et al., 2003a; Vandamme et al., 2003), opening up the possibility of calcium signal conduction along brain blood vessels. Endothelial calcium signals can, in principle, be further communicated to the immune cells in the blood: both gap junction channels and connexin hemichannels have been implicated in such communication system which has been dubbed ‘the immunological synapse’ (Oviedo-Orta and

K. Braet et al. / Biology of the Cell 96 (2004) 79–91

Evans, 2002) and the target immune cells express the necessary purinergic receptors to respond to endothelially released ATP (Di Virgilio et al., 2001). Endothelial cells release large amounts (1-2 % of the total cell content) of ATP in response to a single InsP3 stimulus (Braet et al., 2003c), possibly to overcome the diluting effect of the blood flow. In this context, connexin hemichannels offer an attractive option as a pathway for fast and massive ATP release towards the blood. Finally, endothelial calcium signals might communicate back to the astrocytes (Braet et al., 2001; Leybaert et al., 1998) via the pathways discussed, and might also indirectly affect the neural tissue: work originally performed in the liver demonstrated that endothelially released NO can modulate calcium signal communication in surrounding cells (Charles, 1999), an observation that stresses the need to incorporate the microvascular cells in a picture of integrated neural tissue functioning. Several possibilities should be considered concerning the role of astrocyte-endothelial calcium signal communication. Changes of endothelial calcium are considered a key step in disrupting the tight junctions between endothelial cells and opening of the blood-brain barrier (Revest et al., 1991; Abbott, 1998; Mayhan, 2001; Tiruppathi et al., 2002) and astrocyte-endothelial calcium signals might thus be involved in the process of barrier opening under pathological conditions. A fundamental question is whether endothelial calcium signals have effects on the transports occurring over the blood-brain barrier. We have put forward the hypothesis that astrocyte-endothelial calcium signals are instrumental in what we propose to call ‘neurobarrier coupling’ which, in concerted action with neurovascular and neurometabolic coupling, contributes to adapting the transport of glucose over the barrier to the local astrocytic and neuronal needs. There have been reports on stimulation of the GLUT-1 glucose transporter in response to a calcium challenge with ionophores (Mitani et al., 1995), but clearly, more physiologic calcium changes are needed to investigate this option. Preliminary work suggests that certain neurotransmitters acting on endothelial calcium are able to stimulate glucose uptake in endothelial cells (Braet and Leybaert, 2000) and recent work by Loaiza et al. (2003) has explored this possibility at the level of astrocytes where glutamate was shown to stimulate glucose uptake. The effect of glutamate on endothelial cells need to be determined but there is controversy whether endothelial glutamate receptors are really functional (Morley et al., 1998; Krizbai et al., 1998; Parfenova et al., 2003). Recent work indicates that glutamate can induce carbon monoxide release from brain vessel endothelial cells (Leffler et al., 2003). Astrocytes are, with their typical endfeet, also in contact with arteriolar smooth muscle cells and calcium signals between these two cell types may be a crucial signaling step in neurovascular coupling, i.e. adapting the blood flow to the local neuronal demands (Fig. 1). Recently, Zonta et al. (2003) have used brain slices to demonstrate that astrocytic calcium signals induced by electric stimulation of the neural tissue

85

dilates small vessels in the vicinity through prostaglandindependent signaling. The role of calcium changes in target endothelial or smooth muscle cells was however not investigated (Anderson and Nedergaard, 2003). In acute brain slices, we observed responses in smooth muscle cells following electrically triggered astrocytic calcium waves (unpublished observation). Instead of a direct communication pathway between astrocytes and smooth muscle cells, endothelial cells might act as an intermediary unit in the signaling cascade. Released substances from endothelial cells can either elicit cerebral artery dilation or contraction (Kis et al., 1999). Among the best known vasoconstrictory molecules are endothelins and the prostaglandin PGF2a, both being released in response to a calcium increase and able to induce calciummediated contraction of smooth muscle cells (Saito et al., 1989; Neylon, 1999). The release of vasodilating agents such as NO and the prostaglandins PGE2 and PGI2 is also triggered by a calcium increase (Luckhoff et al., 1988), but their relaxating effect on smooth muscle cells relies on a decrease1 of cytoplasmic calcium levels in response to cGMP (Griffith et al., 1985; Goligorsky, 1988; Lee et al., 1990). In addition to the former agents, vasoconstriction can also be induced by hyperpolarization of the smooth muscle cells, a response attributed to the calcium-dependent secretion of endothelium-derived hyperpolarizing factor (EDHF) (Chen and Suzuki, 1990). Calcium signal communication has furthermore been reported in the inverse direction, i.e. from smooth muscle to endothelial cells, a signal that is proposed to be mediated by gap junctions between both cell types and implicated in a feedback loop involving endothelial nitric oxide generation and smooth muscle cell relaxation (Dora et al., 1997). An attractive hypothesis is that endothelial cells communicate calcium signals between each other from the capillary level to the arterioles, to cause dilation of upstream located resistance vessels (Iadecola, 1993; Faraci and Heistad, 1998). Capillary-to-arteriole endothelial signaling has been demonstrated outside the brain where the communicated signal appears to be electrical for long range signaling and possibly calcium for short range signaling (Sarelius et al., 2000; Dora et al., 2003). 3.4. Calcium signals between astrocytes and microglia Microglial cells are small ramified cells in the brain that act as resident macrophages that become activated under pathological conditions. The first evidence for calciummediated intercellular signaling between astrocytes and microglia in co-culture was provided by Verderio and Matteoli (2001). In addition, stimulation of astrocytes in brain slices evoked calcium waves that spread to neighboring microglia (Schipke et al., 2002). In both conditions, ATP was demonstrated to bring the calcium message from astrocytes to 1 Translation of a calcium increase in one cell to a calcium decrease in another cell is also considered as calcium signal communication, in analogy to the EPSP’s and IPSP’s of electrical signal communication.

86

K. Braet et al. / Biology of the Cell 96 (2004) 79–91

microglial cells by acting on P2X7 receptors. Several suggestions have been made concerning the function of astrocytemicroglial signals, again either related to the calcium signal or to the ATP messenger, including microglial activation (Inoue, 2002), control of the number of microglial cells under pathophysiological conditions (Verderio and Matteoli, 2001) or a signal leading to microglial ramification and cell death (Wollmer et al., 2001). An interesting option is that stimulation of microglial purinergic receptors leads to the release of the cytokine IL-1b (Sanz and Di Virgilio, 2000) which reduces gap junctional communication and potentiates paracrine communication of calcium signals between astrocytes (John et al., 1999), a pathway through which microglia might modulate astrocyte communication. The ATP messenger does not have to come from astrocytes only, as neuronal ATP have also been reported to affect microglia and induce chemotaxis (Honda et al., 2001; Kanazawa et al., 2002). Another possible messenger is platelet-activating factor (PAF), which is released by neurons in a calcium-dependent manner and acts on microglial cells to increase calcium by mobilization from stores and entry form the extracellular space, a signal that again induces chemotaxis (Righi et al., 1995; Aihara et al., 2000). 3.5. Calcium signals between astrocytes and meningeal cells Meningeal cells constitute the three connective tissue membranes surrounding the brain, i.e. the pia mater, the arachnoidea and the dura mater. Bidirectional calcium signaling has been described between astrocytes and meningeal cells from the pia-arachnoid (Grafstein et al., 2000), where both the involvement of gap junctions and a purinergic mechanism have been implicated. Anatomically, a meningoglial network has been demonstrated (Mercier and Hatton, 2000) but in vivo work is necessary to confirm astrocytemenigeal calcium signaling.

4. Concluding remarks It is clear that calcium signals are communicated between various cell types within the brain, with neuronal-glial calcium signal communication being the best defined system and glial-vascular signaling needing further exploration at the in situ level. Calcium signal communication systems show remarkable diversity at the level of messengers and redundancy at the level of pathways. Calcium ions are, however, definitely not the ‘passe-partout’ solution to all our signaling questions and in neurons and vascular cells calcium intimately interacts with electrical signaling, in addition to many other modulating influences not considered in this review. The point that should however emerge is that the classical brain cells like neurons and glial cells should be considered together with the vascular cells to which they are intimately related both at the level of organization and func-

tion of the neural tissue. Further work will be needed to determine the role of the various communication pathways operating in the three party system of neurons, glial cells and vascular cells that together constitute the basic functional unit of the central nervous system.

Acknowledgements Research supported by the Fund for Scientific Research Flanders, Belgium (FWO, grant nos. 3G023599, 3G001201, G.0335.03 and a grant for a long stay abroad to L.L.), the Belgian Society for Scientific Research in Multiple Sclerosis (WOMS, grant no. 51F06700 to L.L.), Ghent University (BOF, grant nos. 01115099, 01107101 and 01113403 to L.L.) and the Queen Elisabeth Medical Foundation (GSKE, grant no. 365B5602 to L.L.).

References Abbott, N.J., 1998. Role of intracellular calcium in regulation of brain endothelial permeability. In: Pardridge, W.M. (Ed.), Introduction to the Blood-Brain Barrier: Methodology, Biology and Pathology. Cambridge University Press, Cambridge, UK, pp. 345–353. Aihara, M., Ishii, S., Kume, K., Shimizu, T., 2000. Interaction between neurone and microglia mediated by platelet-activating factor. Genes Cells 5, 397–406. Allbritton, N.L., Meyer, T., Stryer, L., 1992. Range of messenger action of calcium ion and inositol 1,4,5- trisphosphate. Science 258, 1812–1815. Alvarez-Maubecin, V., Garcia-Hernandez, F., Williams, J.T., Van Bockstaele, E.J., 2000. Functional coupling between neurons and glia. J Neurosci 20, 4091–4098. Anderson, C.M., Nedergaard, M., 2003. Astrocyte-mediated control of cerebral microcirculation. Trends Neurosci 26, 340–344. Araque, A., Carmignoto, G., Haydon, P.G., 2001. Dynamic signaling between astrocytes and neurons. Annu Rev Physiol 63, 795–813. Araque, A., Parpura, V., Sanzgiri, R.P., Haydon, P.G., 1998a. Glutamatedependent astrocyte modulation of synaptic transmission between cultured hippocampal neurons. Eur J Neurosci 10, 2129–2142. Araque, A., Sanzgiri, R.P., Parpura, V., Haydon, P.G., 1998b. Calcium elevation in astrocytes causes an NMDA receptor-dependent increase in the frequency of miniature synaptic currents in cultured hippocampal neurons. J Neurosci 18, 6822–6829. Araque, A., Parpura, V., Sanzgiri, R.P., Haydon, P.G., 1999a. Tripartite synapses: glia, the unacknowledged partner. Trends Neurosci 22, 208– 215. Araque, A., Sanzgiri, R.P., Parpura, V., Haydon, P.G., 1999b. Astrocyteinduced modulation of synaptic transmission. Can J Physiol Pharmacol 77, 699–706. Araque, A., Li, N., Doyle, R. T., Haydon, P.G., 2000. SNARE proteindependent glutamate release from astrocytes. J Neurosci 20, 666–673. Araque, A., Martin, E.D., Perea, G., Arellano, J. I., Buno, W., 2002. Synaptically released acetylcholine evokes Ca2+ elevations in astrocytes in hippocampal slices. J Neurosci 22, 2443–2450. Arcuino, G., Lin, J.H., Takano, T., Liu, C., Jiang, L., Gao, Q., Kang, J., Nedergaard, M., 2002. Intercellular calcium signaling mediated by point-source burst release of ATP. Proc Natl Acad Sci USA 99, 9840– 9845. Attwell, D., 1994. Glia and neurons in dialogue. Nature 369, 707–708.

K. Braet et al. / Biology of the Cell 96 (2004) 79–91 Augustine, G.J., 2001. How does calcium trigger neurotransmitter release ? Curr Opin Neurobiol 11, 320–326. Augustine, G.J., Santamaria, F., Tanaka, K., 2003. Local calcium signaling in neurons. Neuron 40, 331–346. Bading, H., Segal, M., Sucher, N.J., Dudek, H., Lipton, S.A., Greenberg, M.E., 1995. N-methyl-D-aspartate receptors are critical for mediating the effects of glutamate on intracellular calcium concentration and immediate early gene expression in cultured hippocampal neurons. Neuroscience 64, 653–664. Ballerini, P., Di Iorio, P., Ciccarelli, R., Nargi, E., D’Alimonte, I., Traversa, U., Rathbone, M.P., Caciagli, F., 2002. Glial cells express multiple ATP binding cassette proteins which are involved in ATP release. Neuroreport 13, 1789–1792. Bal-Price, A., Moneer, Z., Brown, G.C., 2002. Nitric oxide induces rapid, calcium-dependent release of vesicular glutamate and ATP from cultured rat astrocytes. Glia 40, 312–323. Basarsky, T.A., Duffy, S.N., Andrew, R.D., MacVicar, B.A., 1998. Imaging spreading depression and associated intracellular calcium waves in brain slices. J Neurosci 18, 7189–7199. Berridge, M.J., 2002. The endoplasmic reticulum: a multifunctional signaling organelle. Cell Calcium 32, 235–249. Berridge, M.J., Lipp, P., Bootman, M.D., 2000. The versatility and universality of calcium signalling. Nat Rev Mol Cell Biol 1, 11–21. Bezzi, P., Volterra, A., 2001. A neuron-glia signalling network in the active brain. Curr Opin Neurobiol 11, 387–394. Bezzi, P., Domercq, M., Vesce, S., Volterra, A., 2001. Neuron-astrocyte cross-talk during synaptic transmission: physiological and neuropathological implications. Prog Brain Res 132, 255–265. Bezzi, P., Carmignoto, G., Pasti, L., Vesce, S., Rossi, D., Rizzini, B.L., Pozzan, T., Volterra, A., 1998. Prostaglandins stimulate calciumdependent glutamate release in astrocytes. Nature 391, 281–285. Bodin, P., Burnstock, G., 2001. Evidence that release of adenosine triphosphate from endothelial cells during increased shear stress is vesicular. J Cardiovasc Pharmacol 38, 900–908. Boitano, S., Dirksen, E.R., Sanderson, M.J., 1992. Intercellular propagation of calcium waves mediated by inositol trisphosphate. Science 258, 292– 295. Bowman, C.L., Yohe, L., Lohr, J.W., 2001. Regulation of cytoplasmic calcium levels by two nitric oxide receptors. Am J Physiol Cell Physiol 281, C876–C885. Braet, K., Leybaert, L., 2000. Acute stimulation of glucose transport in endothelial cells by ATP and histamine. Eur J Neurosci 12 (suppl S2000), 356. Braet, K., Paemeleire, K., D’Herde, K., Sanderson, M.J., Leybaert, L., 2001. Astrocyte-endothelial cell calcium signals conveyed by two signalling pathways. Eur J Neurosci 13, 79–91. Braet, K., Vandamme, W., Martin, P., Evans, H., Leybaert, L., 2003a. Photoliberating inositol-1,4,5-trisphosphate triggers ATP release that is blocked by the connexin mimetic peptide gap 26. Cell Calcium 33, 37–48. Braet, K., Vandamme, W., Martin, P.E.M., Evans, W.H., Leybaert, L., 2003b. Pharmacological sensitivity of ATP release triggered by photoliberating InsP3 or by reduced extracellular calcium in brain endothelial cells. J Cell Physiol 197, 205–213. Braet, K., Mabilde, C., Cabooter, L., Rapp, G., Leybaert, L., 2003c. Electroporation loading and photoactivation of caged InsP3: tools to investigate the relation between cellular ATP release in response to intracellular InsP3 elevation. J Neurosci Methods in press. Charles, A., 1998. Intercellular calcium waves in glia. Glia 24, 39–49. Charles, A., 1999. Nitric oxide pumps up calcium signalling. Nat Cell Biol 1, E193–E195. Charles, A.C., 1994. Glia-neuron intercellular calcium signaling. Dev Neurosci 16, 196–206.

87

Charles, A.C., Dirksen, E.R., Merrill, J.E., Sanderson, M.J., 1993. Mechanisms of intercellular calcium signaling in glial cells studied with dantrolene and thapsigargin. Glia 7, 134–145. Charles, A.C., Kodali, S.K., Tyndale, R.F., 1996. Intercellular calcium waves in neurons. Mol Cell Neurosci 7, 337–353. Charles, A.C., Merrill, J.E., Dirksen, E.R., Sanderson, M.J., 1991. Intercellular signaling in glial cells: calcium waves and oscillations in response to mechanical stimulation and glutamate. Neuron 6, 983–992. Charles, A.C., Naus, C.C., Zhu, D., Kidder, G.M., Dirksen, E.R., Sanderson, M.J., 1992. Intercellular calcium signaling via gap junctions in glioma cells. J Cell Biol 118, 195–201. Chen, G.F., Suzuki, H., 1990. Calcium dependency of the endotheliumdependent hyperpolarization in smooth muscle cells of the rabbit carotid artery. J Physiol 421, 521–534. Churchill, G., Louis, C., 1998. Roles of Ca2+, inositol trisphosphate and cyclic ADP-ribose in mediating intercellular Ca2+ signaling in sheep lens cells. J Cell Sci 111, 1217–1225. Clair, C., Chalumeau, C., Tordjmann, T., Poggioli, J., Erneux, C., Dupont, G., Combettes, L., 2001. Investigation of the roles of Ca(2+) and InsP(3) diffusion in the coordination of Ca(2+) signals between connected hepatocytes. J Cell Sci 114, 1999–2007. Coco, S., Calegari, F., Pravettoni, E., Pozzi, D., Taverna, E., Rosa, P., Matteoli, M., Verderio, C., 2003. Storage and release of ATP from astrocytes in culture. J Biol Chem 278, 1354–1362. Cornell-Bell, A.H., Finkbeiner, S.M., Cooper, M.S., Smith, S.J., 1990. Glutamate induces calcium waves in cultured astrocytes: long-range glial signaling. Science 247, 470–473. Cotrina, M.L., Lin, J.H., Nedergaard, M., 1998a. Cytoskeletal assembly and ATP release regulate astrocytic calcium signaling. J Neurosci 18, 8794– 8804. Cotrina, M.L., Lin, J.H., Alves-Rodrigues, A., Liu, S., Li, J., AzmiGhadimi, H., Kang, J., Naus, C.C., Nedergaard, M., 1998b. Connexins regulate calcium signaling by controlling ATP release. Proc Natl Acad Sci USA 95, 15735–15740. Dani, J.W., Chernjavsky, A., Smith, S.J., 1992. Neuronal activity triggers calcium waves in hippocampal astrocyte networks. Neuron 8, 429–440. Darby, M., Kuzmiski, J.B., Panenka, W., Feighan, D., MacVicar, B.A., 2003. ATP released from astrocytes during swelling activates chloride channels. J Neurophysiol 89, 1870–1877. De Pina-Benabou, M.H., Srinivas, M., Spray, D.C., Scemes, E., 2001. Calmodulin kinase pathway mediates the K+-induced increase in Gap junctional communication between mouse spinal cord astrocytes. J Neurosci 21, 6635–6643. Dermietzel, R., Spray, D.C., 1993. Gap junctions in the brain: where, what type, how many and why? Trends Neurosci 16, 186–192. Di Virgilio, F., Borea, P.A., Illes, P., 2001. P2 receptors meet the immune system. Trends Pharmacol Sci 22, 5–7. Domenighetti, A.A., Beny, J.L., Chabaud, F., Frieden, M., 1998. An intercellular regenerative calcium wave in porcine coronary artery endothelial cells in primary culture. J Physiol 513 (Pt 1), 103–116. Dora, K.A., Doyle, M.P., Duling, B.R., 1997. Elevation of intracellular calcium in smooth muscle causes endothelial cell generation of NO in arterioles. Proc Natl Acad Sci USA 94, 6529–6534. Dora, K.A., Xia, J., Duling, B.R., 2003. Endothelial cell signaling during conducted vasomotor responses. Am J Physiol Heart Circ Physiol 285, H119–H126. Duan, S., Anderson, C.M., Keung, E.C., Chen, Y., Swanson, R.A., 2003. P2X7 receptor-mediated release of excitatory amino acids from astrocytes. J Neurosci 23, 1320–1328. Duffy, H.S., Delmar, M., Spray, D.C., 2002. Formation of the gap junction nexus: binding partners for connexins. J Physiol Paris 96, 243–249. Evans, W.H., Martin, P.E., 2002. Gap junctions: structure and function (Review). Mol Membr Biol 19, 121–136.

88

K. Braet et al. / Biology of the Cell 96 (2004) 79–91

Fam, S.R., Gallagher, C.J., Kalia, L.V., Salter, M.W., 2003. Differential Frequency Dependence of P2Y1- and P2Y2- Mediated Ca 2+ Signaling in Astrocytes. J Neurosci 23, 4437–4444.

Hofer, T., Venance, L., Giaume, C., 2002. Control and plasticity of intercellular calcium waves in astrocytes: a modeling approach. J Neurosci 22, 4850–4859.

Faraci, F.M., Heistad, D.D., 1998. Regulation of the cerebral circulation: role of endothelium and potassium channels. Physiol Rev 78, 53–97.

Honda, S., Sasaki, Y., Ohsawa, K., Imai, Y., Nakamura, Y., Inoue, K., Kohsaka, S., 2001. Extracellular ATP or ADP induce chemotaxis of cultured microglia through Gi/o-coupled P2Y receptors. J Neurosci 21, 1975–1982. Iadecola, C., 1993. Regulation of the cerebral microcirculation during neural activity: is nitric oxide the missing link? Trends Neurosci 16, 206–214. Innocenti, B., Parpura, V., Haydon, P.G., 2000. Imaging extracellular waves of glutamate during calcium signaling in cultured astrocytes. J Neurosci 20, 1800–1808. Inoue, K., 2002. Microglial activation by purines and pyrimidines. Glia 40, 156–163. James, G., Butt, A.M., 2001. P2X and P2Y purinoreceptors mediate ATPevoked calcium signalling in optic nerve glia in situ. Cell Calcium 30, 251–259. Jeremic, A., Jeftinija, K., Stevanovic, J., Glavaski, A., Jeftinija, S., 2001. ATP stimulates calcium-dependent glutamate release from cultured astrocytes. J Neurochem 77, 664–675. John, G.R., Scemes, E., Suadicani, S.O., Liu, J.S., Charles, P.C., Lee, S.C., Spray, D.C., Brosnan, C. F., 1999. IL-1beta differentially regulates calcium wave propagation between primary human fetal astrocytes via pathways involving P2 receptors and gap junction channels. Proc Natl Acad Sci USA 96, 11613–11618. Kaftan, E.J., Ehrlich, B.E., Watras, J., 1997. Inositol 1,4,5-trisphosphate (InsP3) and calcium interact to increase the dynamic range of InsP3 receptor-dependent calcium signaling. J Gen Physiol 110, 529–538. Kanazawa, H., Ohsawa, K., Sasaki, Y., Kohsaka, S., Imai, Y., 2002. Macrophage/microglia-specific protein Iba1 enhances membrane ruffling and Rac activation via phospholipase C-gamma -dependent pathway. J Biol Chem 277, 20026–20032. Kang, J., Jiang, L., Goldman, S.A., Nedergaard, M., 1998. Astrocytemediated potentiation of inhibitory synaptic transmission. Nat Neurosci 1, 683–692. Kettenmann, H., Schipke, C.G., 2003. Calcium signaling in glia. In: Hatton, G.I., Parpura, V. (Eds.), Glial/neuronal signaling. Kluwer, Boston in press. Khakh, B.S., Burnstock, G., Kennedy, C., King, B.F., North, R.A., Seguela, P., Voigt, M., Humphrey, P.P., 2001. International union of pharmacology: XXIV, current status of the nomenclature and properties of P2X receptors and their subunits. Pharmacol Rev 53, 107–118. King, B.F., Burnstock, G., Boyer, J.L., Boeynaems, J.M., Weisman, G.A., Kennedy, C., Jacobson, K.A., Humphries, R.G., Abbracchio, M.P., Gachet, C., Miras-Portugal, M.T., 2000. The P2Y receptors, The IUPHAR compendium of receptor characterization and classification. IUPHAR Media, London, pp. 307–320. Kis, B., Szabo, C.A., Pataricza, J., Krizbai, I.A., Mezei, Z., Gecse, A., Telegdy, G., Papp, J.G., Deli, M.A., 1999. Vasoactive substances produced by cultured rat brain endothelial cells. Eur J Pharmacol 368, 35–42. Koizumi, S., Saito, Y., Nakazawa, K., Nakajima, K., Sawada, J.I., Kohsaka, S., Illes, P., Inoue, K., 2002. Spatial and temporal aspects of Ca2+ signaling mediated by P2Y receptors in cultured rat hippocampal astrocytes. Life Sci 72, 431–442. Korkotian, E., Segal, M., 1998. Fast confocal imaging of calcium released from stores in dendritic spines. Eur J Neurosci 10, 2076–2084. Krizbai, I.A., Deli, M.A., Pestenacz, A., Siklos, L., Szabo, C.A., Andras, I., Joo, F., 1998. Expression of glutamate receptors on cultured cerebral endothelial cells. J Neurosci Res 54, 814–819. Lazarowski, E.R., Homolya, L., Boucher, R.C., Harden, T.K., 1997. Direct demonstration of mechanically induced release of cellular UTP and its implication for uridine nucleotide receptor activation. J Biol Chem 272, 24348–24354.

Fields, R.D., Stevens-Graham, B., 2002. New insights into neuron-glia communication. Science 298, 556–562. Finch, E.A., Augustine, G.J., 1998. Local calcium signalling by inositol1,4,5-trisphosphate in Purkinje cell dendrites. Nature 396, 753–756. Froes, M.M., Correia, A.H., Garcia-Abreu, J., Spray, D.C., Campos de Carvalho, A.C., Neto, M.V., 1999. Gap-junctional coupling between neurons and astrocytes in primary central nervous system cultures. Proc Natl Acad Sci USA 96, 7541–7546. Fumagalli, M., Brambilla, R., D’Ambrosi, N., Volonte, C., Matteoli, M., Verderio, C., Abbracchio, M.P., 2003. Nucleotide-mediated calcium signaling in rat cortical astrocytes: Role of P2X and P2Y receptors. Glia 43, 218–1203. Galione, A., White, A., Willmott, N., Turner, M., Potter, B.V., Watson, S.P., 1993. cGMP mobilizes intracellular Ca2+ in sea urchin eggs by stimulating cyclic ADP-ribose synthesis. Nature 365, 456–459. Gallagher, C.J., Salter, M.W., 2003. Differential properties of astrocyte calcium waves mediated by P2Y1 and P2Y2 receptors. J Neurosci 23, 6728–6739. Gallo, V., Ghiani, C.A., 2000. Glutamate receptors in glia: new cells, new inputs and new functions. Trends Pharmacol Sci 21, 252–258. Giaume, C., Venance, L., 1998. Intercellular calcium signaling and gap junctional communication in astrocytes. Glia 24, 50–64. Glaum, S.R., Holzwarth, J.A., Miller, R.J., 1990. Glutamate receptors activate Ca2+ mobilization and Ca2+ influx into astrocytes. Proc Natl Acad Sci USA 87, 3454–3458. Goligorsky, M.S., 1988. Mechanical stimulation induces Ca2+i transients and membrane depolarization in cultured endothelial cells. Effects on Ca2+i in co- perfused smooth muscle cells. FEBS Lett 240, 59–64. Goodenough, D.A., Paul, D.L., 2003. Beyond the gap: functions of unpaired connexon channels. Nat Rev Mol Cell Biol 4, 285–294. Goodenough, D.A., Goliger, J.A., Paul, D.L., 1996. Connexins, connexons, and intercellular communication. Annu Rev Biochem 65, 475–502. Grafstein, B., Liu, S., Cotrina, M.L., Goldman, S.A., Nedergaard, M., 2000. Meningeal cells can communicate with astrocytes by calcium signaling. Ann Neurol 47, 18–25. Griffith, T.M., Edwards, D.H., Lewis, M.J., Henderson, A.H., 1985. Evidence that cyclic guanosine monophosphate (cGMP) mediates endothelium-dependent relaxation. Eur J Pharmacol 112, 195–202. Guthrie, P.B., Knappenberger, J., Segal, M., Bennett, M.V., Charles, A.C., Kater, S.B., 1999. ATP released from astrocytes mediates glial calcium waves. J Neurosci 19, 520–528. Harden, T.K., Lazarowski, E.R., 1999. Release of ATP and UTP from astrocytoma cells. Prog Brain Res 120, 135–143. Harris-White, M.E., Zanotti, S.A., Frautschy, S.A., Charles, A.C., 1998. Spiral intercellular calcium waves in hippocampal slice cultures. Neurophysiol 79, 1045–1052. Hassinger, T.D., Guthrie, P.B., Atkinson, P.B., Bennett, M.V., Kater, S.B., 1996. An extracellular signaling component in propagation of astrocytic calcium waves. Proc Natl Acad Sci USA 93, 13268–13273. Hassinger, T.D., Atkinson, P.B., Strecker, G.J., Whalen, L.R., Dudek, F.E., Kossel, A.H., Kater, S.B., 1995. Evidence for glutamate-mediated activation of hippocampal neurons by glial calcium waves. J Neurobiol 28, 159–170. Haydon, P.G., 2001. GLIA: listening and talking to the synapse. Nat Rev Neurosci 2, 185–193. Hofer, T., Politi, A., Heinrich, R., 2001. Intercellular Ca2+ wave propagation through gap-junctional Ca2+ diffusion: a theoretical study. Biophys J 80, 75–87.

K. Braet et al. / Biology of the Cell 96 (2004) 79–91 Lee, D.K., Faunce, D., Henry, D., Sturm, R.J., Rimele, T., 1990. Neutrophilderived relaxing factor relaxes vascular smooth muscle through a cGMPmediated mechanism. Life Sci 46, 1531–1538. Leffler, C.W., Balabanova, L., Sullivan, C.D., Wang, X., Fedinec, A.L., Parfenova, H., 2003. Regulation of CO production in cerebral microvessels of newborn pigs. Am J Physiol Heart Circ Physiol 285, H292–H297. Leybaert, L., Sanderson, M.J., 2001. Intercellular calcium signaling and flash photolysis of caged compounds. A sensitive method to evaluate gap junctional coupling. Methods Mol Biol 154, 407–430. Leybaert, L., Paemeleire, K., Strahonja, A., Sanderson, M.J., 1998. Inositoltrisphosphate-dependent intercellular calcium signaling in and between astrocytes and endothelial cells. Glia 24, 398–407. Leybaert, L., Braet, K., Vandamme, W., Cabooter, L., Martin, P.E.M., Evans, W.H., 2003. Connexin channels, connexin mimetic peptides and ATP release. Cell Commun Adhes 10 in press. Loaiza, A., Porras, O.H., Barros, L.F., 2003. Glutamate triggers rapid glucose transport stimulation in astrocytes as evidenced by real-time confocal microscopy. J Neurosci 23, 7337–7342. Lowenstein, C.J., Snyder, S.H., 1992. Nitric oxide, a novel biologic messenger. Cell 70, 705–707. Luckhoff, A., Pohl, U., Mulsch, A., Busse, R., 1988. Differential role of extra- and intracellular calcium in the release of EDRF and prostacyclin from cultured endothelial cells. Br J Pharmacol 95, 189–196. Magistretti, P.J., Pellerin, L., 1999. Astrocytes Couple Synaptic Activity to Glucose Utilization in the Brain. News Physiol Sci 14, 177–182. Matyash, V., Filippov, V., Mohrhagen, K., Kettenmann, H., 2001. Nitric oxide signals parallel fiber activity to Bergmann glial cells in the mouse cerebellar slice. Mol Cell Neurosci 18, 664–670. Mayhan, W.G., 2001. Regulation of blood-brain barrier permeability. Microcirculation 8, 89–104. Meir, A., Ginsburg, S., Butkevich, A., Kachalsky, S.G., Kaiserman, I., Ahdut, R., Demirgoren, S., Rahamimoff, R., 1999. Ion channels in presynaptic nerve terminals and control of transmitter release. Physiol Rev 79, 1019–1088. Mercier, F., Hatton, G.I., 2000. Immunocytochemical basis for a meningeoglial network. J Comp Neurol 420, 445–465. Mitani,Y., Behrooz, A., Dubyak, G.R., Ismail-Beigi, F., 1995. Stimulation of GLUT-1 glucose transporter expression in response to exposure to calcium ionophore A-23187. Am J Physiol 269, C1228–C1234. Moerenhout, M., Himpens, B., Vereecke, J., 2001. Intercellular communication upon mechanical stimulation of CPAE- endothelial cells is mediated by nucleotides. Cell Calcium 29, 125–136. Monaghan, D.T., Bridges, R.J., Cotman, C.W., 1989. The excitatory amino acid receptors: their classes, pharmacology, and distinct properties in the function of the central nervous system. Annu Rev Pharmacol Toxicol 29, 365–402. Morley, P., Small, D.L., Murray, C.L., Mealing, G.A., Poulter, M.O., Durkin, J.P., Stanimirovic, D.B., 1998. Evidence that functional glutamate receptors are not expressed on rat or human cerebromicrovascular endothelial cells. J Cereb Blood Flow Metab 18, 396–406. Muller, T., Moller, T., Berger, T., Schnitzer, J., Kettenmann, H., 1992. Calcium entry through kainate receptors and resulting potassiumchannel blockade in Bergmann glial cells. Science 256, 1563–1566. Murphy, S., Simmons, M.L., Agullo, L., Garcia, A., Feinstein, D.L., Galea, E., Reis, D.J., Minc-Golomb, D., Schwartz, J.P., 1993. Synthesis of nitric oxide in CNS glial cells. Trends Neurosci 16, 323–328. Muyderman, H., Angehagen, M., Sandberg, M., Bjorklund, U., Olsson, T., Hansson, E., Nilsson, M., 2001. Alpha 1-adrenergic modulation of metabotropic glutamate receptor-induced calcium oscillations and glutamate release in astrocytes. J Biol Chem 276, 46504–46514. Miyata, M., Finch, E.A., Khiroug, L., Hashimoto, K., Hayasaka, S., Oda, S.I., Inouye, M., Takagishi, Y., Augustine, G.J., Kano, M., 2000. Local calcium release in dendritic spines required for long-term synaptic depression. Neuron 28, 233–244.

89

Nakahara, K., Okada, M., Nakanishi, S., 1997. The metabotropic glutamate receptor mGluR5 induces calcium oscillations in cultured astrocytes via protein kinase C phosphorylation. J Neurochem 69, 1467–1475. Nathan, C., Xie, Q.W., 1994. Nitric oxide synthases: roles, tolls, and controls. Cell 78, 915–918. Nedergaard, M., 1994. Direct signaling from astrocytes to neurons in cultures of mammalian brain cells. Science 263, 1768–1771. Newman, E.A., 1986. High potassium conductance in astrocyte endfeet. Science 233, 453–454. Newman, E.A., 2001. Propagation of intercellular calcium waves in retinal astrocytes and Muller cells. J Neurosci 21, 2215–2223. Newman, E.A., Zahs, K.R., 1998. Modulation of neuronal activity by glial cells in the retina. J Neurosci 18, 4022–4028. Neylon, C.B., 1999. Vascular biology of endothelin signal transduction. Clin Exp Pharmacol Physiol 26, 149–153. Niessen, H., Harz, H., Bedner, P., Kramer, K., Willecke, K., 2000. Selective permeability of different connexin channels to the second messenger inositol 1,4,5-trisphosphate. J Cell Sci 113 (Pt 8), 1365–1372. Nimchinsky, E.A., Sabatini, B.L., Svoboda, K., 2002. Structure and function of dendritic spines. Annu Rev Physiol 64, 313–353. Osipchuk, Y., Cahalan, M., 1992. Cell-to-cell spread of calcium signals mediated by ATP receptors in mast cells. Nature 359, 241–244. Oviedo-Orta, E., Evans, W.H., 2002. Gap junctions and connexins: potential contributors to the immunological synapse. J Leukoc Biol 72, 636–642. Paemeleire, K., Leybaert, L., 2000. ATP-dependent astrocyte-endothelial calcium signaling following mechanical damage to a single astrocyte in astrocyte-endothelial co-cultures. J Neurotrauma 17, 345–358. Parfenova, H., Fedinec, A., Leffler, C.W., 2003. Ionotropic glutamate receptors in cerebral microvascular endothelium are functionally linked to heme oxygenase. J Cereb Blood Flow Metab 23, 190–197. Parpura, V., Haydon, P.G., 2000. Physiological astrocytic calcium levels stimulate glutamate release to modulate adjacent neurons. Proc Natl Acad Sci USA 97, 8629–8634. Parpura, V., Basarsky, T.A., Liu, F., Jeftinija, K., Jeftinija, S., Haydon, P.G., 1994. Glutamate-mediated astrocyte-neuron signalling. Nature 369, 744–747. Parri, H.R., Gould, T.M., Crunelli, V., 2001. Spontaneous astrocytic Ca2+ oscillations in situ drive NMDAR-mediated neuronal excitation. Nat Neurosci 4, 803–812. Paspalas, C.D., Papadopoulos, G.C., 1998. Ultrastructural evidence for combined action of noradrenaline and vasoactive intestinal polypeptide upon neurons, astrocytes, and blood vessels of the rat cerebral cortex. Brain Res Bull 45, 247–259. Pasti, L., Volterra, A., Pozzan, T., Carmignoto, G., 1997. Intracellular calcium oscillations in astrocytes: a highly plastic, bidirectional form of communication between neurons and astrocytes in situ. J Neurosci 17, 7817–7830. Pasti, L., Zonta, M., Pozzan, T., Vicini, S., Carmignoto, G., 2001. Cytosolic calcium oscillations in astrocytes may regulate exocytotic release of glutamate. J Neurosci 21, 477–484. Pearce, B., Albrecht, J., Morrow, C., Murphy, S., 1986. Astrocyte glutamate receptor activation promotes inositol phospholipid turnover and calcium flux. Neurosci Lett 72, 335–340. Pellerin, L., Magistretti, P.J., 1994. Glutamate uptake into astrocytes stimulates aerobic glycolysis: a mechanism coupling neuronal activity to glucose utilization. Proc Natl Acad Sci USA 91, 10625–10629. Perea, G., Araque, A., 2002. Communication between astrocytes and neurons: a complex language. J Physiol Paris 96, 199–207. Petrozzino, J.J., Connor, J.A., 1994. Dendritic Ca2+ accumulations and metabotropic glutamate receptor activation associated with an N-methyl-D-aspartate receptor-independent long-term potentiation in hippocampal CA1 neurons. Hippocampus 4, 546–558. Perkel, D.J., Petrozzino, J.J., Nicoll, R.A., Connor, J.A., 1993. The role of Ca2+ entry via synaptically activated NMDA receptors in the induction of long-term potentiation. Neuron 11, 817–823.

90

K. Braet et al. / Biology of the Cell 96 (2004) 79–91

Phillis, J.W., O’Regan, M.H., 2002. Evidence for swelling-induced adenosine and adenine nucleotide release in rat cerebral cortex exposed to monocarboxylate-containing or hypotonic artificial cerebrospinal fluids. Neurochem Int 40, 629–635. Porter, J.T., McCarthy, K.D., 1995. GFAP-positive hippocampal astrocytes in situ respond to glutamatergic neuroligands with increases in [Ca2+]i. Glia 13, 101–112. Porter, J.T., McCarthy, K.D., 1996. Hippocampal astrocytes in situ respond to glutamate released from synaptic terminals. J Neurosci 16, 5073– 5081. Queiroz, G., Meyer, D.K., Meyer, A., Starke, K., von Kugelgen, I., 1999. A study of the mechanism of the release of ATP from rat cortical astroglial cells evoked by activation of glutamate receptors. Neuroscience 91, 1171–1181. Ralevic, V., Burnstock, G., 1998. Receptors for purines and pyrimidines. Pharmacol Rev 50, 413–492. Revest, P.A., Abbott, N.J., Gillespie, J.I., 1991. Receptor-mediated changes in intracellular [Ca2+] in cultured rat brain capillary endothelial cells. Brain Res 549, 159–161. Reyes-Harde, M., Potter, B.V., Galione, A., Stanton, P.K., 1999. Induction of hippocampal LTD requires nitric-oxide-stimulated PKG activity and Ca2+ release from cyclic ADP-ribose-sensitive stores. J Neurophysiol 82, 1569–1576. Righi, M., Letari, O., Sacerdote, P., Marangoni, F., Miozzo, A., Nicosia, S., 1995. myc-immortalized microglial cells express a functional plateletactivating factor receptor. J Neurochem 64, 121–129. Rottingen, J., Iversen, J.G., 2000. Ruled by waves? Intracellular and intercellular calcium signalling. Acta Physiol Scand 169, 203–219. Rouach, N., Avignone, E., Meme, W., Koulakoff, A., Venance, L., Blomstrand, F., Giaume, C., 2002. Gap junctions and connexin expression in the normal and pathological central nervous system. Biol Cell 94, 457– 475. Rozental, R., Andrade-Rozental, A.F., Zheng, X., Urban, M., Spray, D.C., Chiu, F.C., 2001. Gap junction-mediated bidirectional signaling between human fetal hippocampal neurons and astrocytes. Dev Neurosci 23, 420–431. Saez, J.C., Connor, J.A., Spray, D.C., Bennett, M.V., 1989. Hepatocyte gap junctions are permeable to the second messenger, inositol 1,4,5trisphosphate, and to calcium ions. Proc Natl Acad Sci USA 86, 2708– 2712. Saito, A., Shiba, R., Kimura, S.,Yanagisawa, M., Goto, K., Masaki, T., 1989. Vasoconstrictor response of large cerebral arteries of cats to endothelin, an endothelium-derived vasoactive peptide. Eur J Pharmacol 162, 353– 358. Sanderson, M.J., Charles, A.C., Dirksen, E.R., 1990. Mechanical stimulation and intercellular communication increases intracellular Ca2+ in epithelial cells. Cell Regul 1, 585–596. Sanderson, M.J., Charles, A.C., Boitano, S., Dirksen, E.R., 1994. Mechanisms and function of intercellular calcium signaling. Mol Cell Endocrinol 98, 173–187. Sanz, J.M., Di Virgilio, F., 2000. Kinetics and mechanism of ATP-dependent IL-1 beta release from microglial cells. J Immunol 164, 4893–4898. Sarelius, I.H., Cohen, K.D., Murrant, C.L., 2000. Role for capillaries in coupling blood flow with metabolism. Clin Exp Pharmacol Physiol 27, 826–829. Scemes, E., 2000. Components of astrocytic intercellular calcium signaling. Mol Neurobiol 22, 167–179. Scemes, E., Suadicani, S.O., Spray, D.C., 2000. Intercellular communication in spinal cord astrocytes: fine tuning between gap junctions and P2 nucleotide receptors in calcium wave propagation. J Neurosci 20, 1435– 1445. Schipke, C.G., Boucsein, C., Ohlemeyer, C., Kirchhoff, F., Kettenmann, H., 2002. Astrocyte Ca2+ waves trigger responses in microglial cells in brain slices. Faseb J 16, 255–257.

Schmitz, D., Schuchmann, S., Fisahn, A., Draguhn, A., Buhl, E.H., PetraschParwez, E., Dermietzel, R., Heinemann, U., Traub, R.D., 2001. Axoaxonal coupling. a novel mechanism for ultrafast neuronal communication. Neuron 31, 831–840. Segal, M., 1995. Fast imaging of [Ca]i reveals presence of voltage-gated calcium channels in dendritic spines of cultured hippocampal neurons. J Neurophysiol 74, 484–488. Segal, M., 2002. Dendritic spines: elementary structural units of neuronal plasticity. Prog Brain Res 138, 53–59. Shafer, O.T., Chen, A., Kumar, S.M., Muller, K.J., Sahley, C.L., 1998. Injury-induced expression of endothelial nitric oxide synthase by glial and microglial cells in the leech central nervous system within minutes after injury. Proc R Soc Lond B Biol Sci 265, 2171–2175. Shao, Y., McCarthy, K.D., 1997. Responses of Bergmann glia and granule neurons in situ to N-methyl-D-aspartate, norepinephrine, and high potassium. J Neurochem 68, 2405–2411. Shelton, M.K., McCarthy, K.D., 1999. Mature hippocampal astrocytes exhibit functional metabotropic and ionotropic glutamate receptors in situ. Glia 26, 1–11. Shibata, Y., Kumai, M., Nishii, K., Nakamura, K., 2001. Diversity and molecular anatomy of gap junctions. Med Electron Microsc 34, 153– 159. Simard, M., Arcuino, G., Takano, T., Liu, Q.S., Nedergaard, M., 2003. Signaling at the gliovascular interface. J Neurosci 23, 9254–9262. Sneyd, J., Wetton, B.T., Charles, A.C., Sanderson, M.J., 1995. Intercellular calcium waves mediated by diffusion of inositol trisphosphate: a twodimensional model. Am J Physiol 268, C1537–C1545. Spray, D.C., Dermietzel, R., 1996. Gap junctions in the nervous system. R.G. Landes Company, Austin, Texas. Stout, C.E., Costantin, J.L., Naus, C.C., Charles, A.C., 2002. Intercellular calcium signaling in astrocytes via ATP release through connexin hemichannels. J Biol Chem 14, 14. Suadicani, S.O., De Pina-Benabou, M.H., Urban-Maldonado, M., Spray, D.C., Scemes, E., 2003. Acute downregulation of Cx43 alters P2Y receptor expression levels in mouse spinal cord astrocytes. Glia 42, 160–171. Tiruppathi, C., Minshall, R.D., Paria, B.C., Vogel, S.M., Malik, A.B., 2002. Role of Ca2+ signaling in the regulation of endothelial permeability. Vascul Pharmacol 39, 173–185. Tsacopoulos, M., Poitry-Yamate, C.L., Poitry, S., Perrottet, P., Veuthey, A.L., 1997. The nutritive function of glia is regulated by signals released by neurons. Glia 21, 84–91. Vandamme, W., Braet, K., Cabooter, L., Leybaert, L., 2003. Tumor necrosis factor alpha inhibits purinergic calcium signaling in blood-brain barrier endothelial cells. J Neurochem in press. Venance, L., Stella, N., Glowinski, J., Giaume, C., 1997. Mechanism involved in initiation and propagation of receptor-induced intercellular calcium signaling in cultured rat astrocytes. J Neurosci 17, 1981–1992. Verderio, C., Matteoli, M., 2001. ATP mediates calcium signaling between astrocytes and microglial cells: modulation by IFN-gamma. J Immunol 166, 6383–6391. Verkhratsky, A., Kettenmann, H., 1996. Calcium signalling in glial cells. Trends Neurosci 19, 346–352. Verkhratsky, A., Orkand, R.K., Kettenmann, H., 1998. Glial calcium: homeostasis and signaling function. Physiol Rev 78, 99–141. Verkhratsky, A., Solovyeva, N., Toescu, E.C., 2002. Calcium excitability of glial cells. In: Volterra, A., Magistretti, P., Haydon, P. (Eds.), The Tripartite Synapse - Glia in Synaptic Transmission. Oxford University Press, pp. 99–109. Vesce, S., Bezzi, P., Volterra, A., 2001. Synaptic transmission with the glia. News Physiol Sci 16, 178–184. Volknandt, W., 2002. Vesicular release mechanisms in astrocytic signalling. Neurochem Int 41, 301–306.

K. Braet et al. / Biology of the Cell 96 (2004) 79–91 Volterra, A., Magistretti, P., Haydon, P., 2002. The Tripartite Synapse - Glia in Synaptic Transmission. Oxford University Press. Voutsinos-Porche, B., Bonvento, G., Tanaka, K., Steiner, P., Welker, E., Chatton, J.Y., Magistretti, P.J., Pellerin, L., 2003. Glial glutamate transporters mediate a functional metabolic crosstalk between neurons and astrocytes in the mouse developing cortex. Neuron 37, 275–286. Walz, W., 2000. Role of astrocytes in the clearance of excess extracellular potassium. Neurochem Int 36, 291–300. Wang, Z., Haydon, P.G., Yeung, E.S., 2000. Direct observation of calciumindependent intercellular ATP signaling in astrocytes. Anal Chem 72, 2001–2007. Willmott, N.J., Wong, K., Strong, A.J., 2000a. A fundamental role for the nitric oxide-G-kinase signaling pathway in mediating intercellular Ca(2+) waves in glia. J Neurosci 20, 1767–1779. Willmott, N.J., Wong, K., Strong, A.J., 2000b. Intercellular Ca(2+) waves in rat hippocampal slice and dissociated glial-neuron cultures mediated by nitric oxide. FEBS Lett 487, 239–247. Wollmer, M.A., Lucius, R., Wilms, H., Held-Feindt, J., Sievers, J., Mentlein, R., 2001. ATP and adenosine induce ramification of microglia in vitro. J Neuroimmunol 115, 19–27. Ye, Z.C., Wyeth, M.S., Baltan-Tekkok, S., Ransom, B.R., 2003. Functional hemichannels in astrocytes: a novel mechanism of glutamate release. J Neurosci 23, 3588–3596.

91

Yule, D.I., Stuenkel, E., Williams, J.A., 1996. Intercellular calcium waves in rat pancreatic acini: mechanism of transmission. Am J Physiol 271, C1285–C1294. Yuste, R., Nelson, D.A., Rubin, W.W., Katz, L.C., 1995. Neuronal domains in developing neocortex: mechanisms of coactivation. Neuron 14, 7–17. Zamanillo, D., Sprengel, R., Hvalby, O., Jensen, V., Burnashev, N., Rozov, A., Kaiser, K.M., Koster, H.J., Borchardt, T., Worley, P., Lubke, J., Frotscher, M., Kelly, P.H., Sommer, B., Andersen, P., Seeburg, P.H., Sakmann, B., 1999. Importance of AMPA receptors for hippocampal synaptic plasticity but not for spatial learning. Science 284, 1805–1811. Zhu, Y., Kimelberg, H.K., 2001. Developmental expression of metabotropic P2Y(1) and P2Y(2) receptors in freshly isolated astrocytes from rat hippocampus. J Neurochem 77, 530–541. Zimmermann, H., Braun, N., Kegel, B., Heine, P., 1998. New insights into molecular structure and function of ectonucleotidases in the nervous system. Neurochem Int 32, 421–425. Zonta, M., Carmignoto, G., 2002. Calcium oscillations encoding neuron-toastrocyte communication. J Physiol Paris 96, 193–198. Zonta, M., Angulo, M.C., Gobbo, S., Rosengarten, B., Hossmann, K.A., Pozzan, T., Carmignoto, G., 2003. Neuron-to-astrocyte signaling is central to the dynamic control of brain microcirculation. Nat Neurosci 6, 43–50.